By Tom Rauch, W8JI
Topband Antenna mail list, August
A "magic T"
splitter or combiner is very easy to build, and that's all Mini-Circuits
or a CATV manufacturer will sell you. I often just use a chunk of
old PC board as a "chassis".
1. Wind a bifilar winding on a ferrite core, 73 to 77 mix
will work fine. If the core thickness is about 3/8 inch, you'll
need about five passes of twisted pair wire (any gauge you can work
2. Connect the start of one winding to the finish of the other.
That point becomes the antenna input.
3. The remaining wires become outputs to the receivers. Connect
a resistor that is twice the impedance of the receivers across from
wire to wire. A 100 ohm resistor works for 50 ohm receivers, or
150 ohm for 75 ohm receivers.
4. Feed the center-tap with a 2:1 impedance transformer (a turns
ratio of 5 to 7 will work fine, and that can be an auto-transformer
wound on the same style of core used to make the splitting transformer).
The full winding is the input, the tap point (5 turns off ground)
is the output that drives the center tap of the splitting transformer.
A word of caution, all splitters are sensitive to output
port termination. If one port is misterminated, the output voltage
(and loss through the splitter) will still vary quite a large amount.
The splitter prevents a short or open from totally killing the signal,
but it does not guarantee equal power to each port unless impedances
Turn this around, and it becomes a combiner. About 3dB loss when
matched and splitting, almost a perfect sum of the inputs when combining.